The present invention relates to the transmission of optical pulses over optical fibers and, more particularly, to the compensation of the dispersion that is normally experienced by optical pulses in their travel along optical fibers.
The transmission of optical pulses that carry information over transmission lines has become of major commercial importance. Typically the fibers used for the transmission lines are filaments of high purity silica appropriately protected and they function as waveguides for the optical pulses. Such transmission lines provide transmission of laser pulses with sufficiently low loss so as to permit transmission over relatively long distances without need for regeneration. In many cases the factor that principally limits the total distance the pulses can travel without the need for regeneration is the broadening that the pulses experience during their travel because of the dispersion introduced by the fiber, i.e., pulses of different wavelengths travel with different phase and group velocities along the fiber, and each pulse typically has a broad spectral bandwidth of different frequencies.
Such broadening limits the spacing permissible between pulses and so the capacity of the system, which is related to the number of pulses per time interval that can be transmitted with the high fidelity demanded of many applications. High capacity is of course important to keep unit costs low.
Typically, in an optical fiber transmission line of appreciable multispan length, it is the practice periodically to introduce amplification of the pulses along their path, typically at the end of each span. In the past, the amplification has primarily been provided by discrete amplifiers, generally a relatively short length of erbium-doped fiber that is irradiated with pumping light of appropriate wavelengths so that it acts as an amplifier.
In addition to providing amplification of the optical pulses during their travel, it has become the practice in transmission lines of multispan length to introduce at the upstream end of each span periodic compensation for the dispersion discussed above, essentially to nullify the dispersion expected in the succeeding span. In the past, such compensation has generally taken the form of only partial or undercompensation at the upstream end of each span. The undercompensation at the upstream end has been generally practiced because some pulses tend to exhibit the xe2x80x9csoliton effectxe2x80x9d as a result of self phase modulation, which acts to lessen the broadening effect of the intrinsic dispersion of the fiber.
Systems of this kind have been working well and so have tended to warrant little change.
A recent trend has been to supplement the role of the discrete optical amplifiers used in the past by distributed amplification obtained by xe2x80x9cRamanxe2x80x9d pumping. Such supplemented amplification involves applying pumping light of appropriate wavelength to the fiber for transmission along the fiber, generally for travel upstream opposite to the direction of the signal pulses. Such pumping light can be made to interact with the optical signal pulses to provide amplification by Raman mixing.
However in such systems the use of undercompensation at the upstream end of each span has tended to give mixed results.
The invention is designed to provide a more satisfactory method for compensation for the dispersion involved.
The invention is applicable to an optical transmission system of the typical kind in which the transmission line includes a plurality of spans between the upstream end of the line, where input pulses from a transmitter are supplied, to the downstream end, where the output pulses are received by a receiver for either utilization or regeneration. Such a line typically has included a xe2x80x9cpre-dispersion compensationxe2x80x9d module at the input, or upstream, end of the line, a xe2x80x9cpost-dispersion compensationxe2x80x9d module at the output, or downstream, end of the line and an xe2x80x9cin-line dispersion compensationxe2x80x9d module at each junction between the end of one span and the start of a new span. Each such dispersion compensation module (DCM) generally has been associated with a discrete optical amplifier that generally provides gain to compensate for the attenuation and compensation for the dispersion of the span traveled by the latter, which is generally provided by a section of fiber specially designed to provide a desired dispersion characteristic.
However, in accordance with an exemplary embodiment of the invention, a multispan transmission line of the kind just described would comprise a) an optical fiber, typically with positive dispersion, b) at the input or upstream end a module that provided little if any pre-dispersion compensation, c) at each junction between spans, a module that provided overcompensation of between five and thirty percent of the dispersion experienced in the preceding span, and d) at the output or downstream end, a module that provided the supplemental compensation needed essentially to nullify any remaining uncompensated dispersion of the path, typically undercompensation.
We have discovered that for optimum operation, as measured by the shape of the output pulses, there should be little or no pre-dispersion compensation, and between 110 and 120 percent compensation (10-20 percent overcompensation) of the dispersion of the preceding span at each in-line DCM. In particular, for a transmission line of six spans, each of about 80 km long, the best output pulse shape was obtained with no pre-compensation and 117 percent in-line compensation at each junction between spans. It appears that the larger the number of spans, the closer to the lower edge of the 110-120 percent range the in-line dispersion compensation should be.
Moreover, it also appears that the improvement obtainable is better when the system is also using Raman pumping to balance out the attenuation of the signal as it propagates along the line, the Raman pumping light being directed upstream, opposite in direction to the travel of the signal beam.
To this end, in the preferred embodiments of the invention, there would be included at the end of each span a source of pumping light of appropriate wavelength for Raman mixing amplification of the signal pulse for introducing the light for travel upstream to the start of the span.
It is found that some improvement is available also if the pre-compensation is kept below fifty percent and the in-line compensation at each junction is kept between 105 and 130 percent.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.